EGTA

From OpenWetWare

Ethylene glycol tetraacetic acid (EGTA) is a common buffer ingredient due to its chelating activity. It is similar to the better known EDTA, but has a much higher affinity for calcium ions than for magnesium ions. Buffers made with EGTA are used in some cases to mimic the environment inside living cells where calcium ions are usually at least a thousandfold less concentrated than magnesium ions.